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Proto Board with 12V Power Adapter — Parallax Forums

Proto Board with 12V Power Adapter

Matthew HayMatthew Hay Posts: 63
edited 2007-11-05 16:30 in Propeller 1
Okay the specs list the proto board as using a 6-9VDC power input, the 1086-5V specs list that is can handle up to 20V (I believe). Would it be safe to run the board off a 12V power adapter without adding any type of heat sink?

I think I've got my place of work to buy some proto boards for some projects but I wanted to verify if we could use the abundance of 12V 1Amp power adapters we have sitting around. Also any projects made would be running 24/7 in most likely a sealed enclosures.

Thanks for any help

Comments

  • LeonLeon Posts: 7,620
    edited 2007-11-01 21:39
    The regulator might get hot inside a sealed enclosure, running from 12V. Why not get some 9V supplies, they aren't expensive.

    Leon

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-11-01 21:41
    Please be aware that any so-called 12V adapters capable of supplying 1A probably output a much higher voltage than that under a light load. You might want to measure the actual voltage and reposit your question, given the results of your measurement.

    -Phil
  • deSilvadeSilva Posts: 2,967
    edited 2007-11-01 21:58
    It depends on the current you draw. The datasheet gives the following hint:
    Vin-Vout =20 V -> Imax = 150 mA


    Which means it is not wise to dissipate more than 3W, though this most likely will even need a heatsink.
    Though it is not a proportional calculation, I should say you can easily draw 1W/7V = 150mA @ 12V without a heatsink.

    Note that the total heat inside your box will be 12V*current - absolutely independent of how you use this power. It would be 12W @ 1A which is definitely too much without good airflow and 1.8 W @ 150mA which I should say is acceptable
  • Graham StablerGraham Stabler Posts: 2,510
    edited 2007-11-01 22:14
    I use a demoboard on 12v, OK if you are just talking to a TV/keyboard etc, perhaps not with servos.

    Graham
  • RaymanRayman Posts: 14,793
    edited 2007-11-01 22:31
    I accidentally gave a proto board 12 V (actually ~15V unloaded) today and it survived. What I did is add an extra jack and use a 7809 to go from 12 to 9 V. and then connect the 9 V to normal power jack...
  • Mike GreenMike Green Posts: 23,101
    edited 2007-11-01 22:52
    It's better to start with a 7805 and put 2 or 3 diodes in series with the ground pin. That shifts the regulator voltage upwards by 1.2 to 1.8. That way, most of the power dissipation occurs in the 7805 which you can heatsink to a substantial thermal mass.
  • Matthew HayMatthew Hay Posts: 63
    edited 2007-11-01 23:42
    Okay so long story short it is recommended not to do that.

    Thanks guys.
  • RaymanRayman Posts: 14,793
    edited 2007-11-01 23:47
    True. I think the problem with that rating is that it assumes a very big heat sink is attached...
  • parts-man73parts-man73 Posts: 830
    edited 2007-11-02 02:10
    I looked at the same specs myself when I received my first Proto Board. I couldn't find my 9V wallwart, so I plugged it into my laptop charger (18V) briefly after reading the same specs you spoke of. No hard came to it. But I just quickly ran the VGA demo to verify it worked. The 5volt regulator got pretty hot in the brief time I had it on though, I wouldn't recommend anything higher than 9 volts if you are using any substantial current.

    Case in point, when I plug in my PropNIC, with a 9 volt wallwart, the regulator gets very hot quickly. The ENC28j60 chip itself draws 250mA. When I use a 6 volt supply, the regulator barely gets warm.

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  • deSilvadeSilva Posts: 2,967
    edited 2007-11-02 05:59
    Please, boys: State your current! It is total poppycock to say: "Oh, it works at my board..."
    It works with 30V of course, when you draw just 100mA!!

    To remind you:
    V = R*I Ohm's Law
    P = V*I Power dissipation
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-02 07:46
    Sometimes I think this group needs an electronics 101. eyes.gif There are lots of opinions but few facts although they are easy to find.

    As you said DeSilva, basic high school Ohm's law stuff (and from there power calculations) although 30V@100ma is a fire starter!

    Doesn't anyone use switch-mode regulators anyway? 5V@1A from an 8-pin smd running from 30V with no heatsink. hop.gif

    *Peter*
  • Nick MuellerNick Mueller Posts: 815
    edited 2007-11-02 08:48
    Or switching regulators in an almost TO220 case, pin-compatible with the 78XX linear regulators (for those who can't design a switching regulator).


    Nick

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  • JavalinJavalin Posts: 892
    edited 2007-11-02 09:05
    I've modified one of mine - the only issue I could see was the 10V cap on the VIN side of the regulator.

    With care you can un-solder and replace with a 50V cap....

    J
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-02 09:51
    Just as an exercise I did a quick switch-mode design in a compact 3-pin module, just like a 7805.

    See, that wasn't hard.

    *Peter*
    SM7805.jpg
    545 x 312 - 33K
    SM7805 SCH.jpg
    694 x 399 - 28K
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-11-02 18:11
    Switch-mode wall transformers are dirt cheap now, too. Six-watt regulated-output units from CUI can be had from DigiKey for less than $6.00.

    -Phil
  • deSilvadeSilva Posts: 2,967
    edited 2007-11-02 18:31
    Peter Jakacki said...
    although 30V@100ma is a fire starter!
    I just quoted the data sheet - report to them smile.gif
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-02 23:54
    DeSilva:
    Ahhh, the datasheet, yes. What's missing from the datasheets is the safe operating area region (SOAR) information but understandably that gets complicated with all the variable conditions. Still, the datasheet provides the device characteristics which via some basic maths gives us an idea whether it is up to the task. The graphs are also very helpful and reading the footnotes should be mandatory.

    Phil:
    Aren't those things so cheap! Still, they have a habit of giving you a bite due to the cap between the mains neutral and dc common. Anyway many times commercial equipment has to operate from a system supply of +12 or +24V and plug packs (that's what we call them) in these installations are troublesome, they take up room, require a mains connection, need to be secured (whoops) etc.

    *Peter*
  • deSilvadeSilva Posts: 2,967
    edited 2007-11-02 23:57
    LOL - That was nearly word by word what I explaind about data sheets 6 hours ago smile.gif
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-03 00:26
    deSilva said...
    LOL - That was nearly word by word what I explaind about data sheets 6 hours ago smile.gif
    deSilva previously said said...
    I just quoted the data sheet - report to them

    To paraphrase a proverb "if you say enough you're sure to stuff up" tongue.gif

    ..and if we didn't say much then what fun would that be?


    I tidied up my little switcher pcb and made it smaller. I will probably do a run of them off my next proto panel. They are so small that I'd probably get 50 just filling up the spaces. On the switcher module I have allowed for a small trim resistor to adjust the voltage plus I may dip the modules in a conformal coat. They're not really any larger than a 7805 TO220 pack plus they don't need a heatsink and run about 80% efficient.

    *Peter*
    SM7805A.jpg
    714 x 432 - 60K
  • RaymanRayman Posts: 14,793
    edited 2007-11-03 00:26
    I'm contemplating a solar powered application...· Probably need to go from 3 or 4 AA NiCads to 3.3 Volts.

    Wonder if Peter's switch mode supply would save battery life...· I'm a little discouraged by how hot the 3.3 V regulator on the Proto board gets...
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-03 01:46
    Rayman:
    Message me your address and I will send some freebies to you in a week or two.

    *Peter*
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-05 04:53
    Some members have expressed interest in the little switch-mode module that I mentioned plus one asked for 3.3V output as well. I am making these modules available at a very attractive price (cost+). Besides the 3-pin module I also have a 7-pin SIL module that features +5V@500ma switch-mode as well as a 3.3V @250ma LDO output with 500ma combined output current. There will also be a 1A version but the 0.5A part is cheaper. For propheads I am trying to keep the cost of these regulators under $10ea in quantities of 10 off.

    *Peter*
    SM7805+3.jpg
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  • Ken PetersonKen Peterson Posts: 806
    edited 2007-11-05 05:05
    We're using the proto board with 12V supply, and we have found the regulators to get hot, just running a couple of motor drivers. The drivers are running for the 12V input and not through the regulators. I understand that the regulators have built-in thermal shut down.

    We're not using a wall wart, as our application is for automotive. So we put an extra 7808 regulator in there with a big heat sink. No problems.

    I still don't understand the design decision to feed to the 3.3V regulator from the 5V regulator. For another project, I designed a board to use the propeller and I also had to use dual voltages because I had video switchers that run on 5V. I used a 317 regulator for each voltage and each is fed from the 12V input. No problems here even without heat sinks.

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    The more I know, the more I know I don't know.· Is this what they call Wisdom?
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-11-05 05:54
    Pre-regulation, especially when a linear regulator follows a switcher is not an uncommon design practice (I use it all the time). It is very inefficient to run the 3.3V regulator direct from the +12V supply as you are now dissipating approx 9*x amps in power which means close to 1W for 100ma. Also, many LDO 3.3V regulators are not spec'd for high input voltages of +12V. The switcher provides an efficient step-down to +5V from any higher voltage up to +35V. The 3.3V LDO regulator is happy as it only has to dissipate 1.7*x amps (170mw at 100ma) so you can run the regulator to it's maximum output current without undue heat being dissipated. The pre-regulator also conditions the input voltage to the regulator (they're not perfect regulators) resulting in a cleaner supply for the prop etc.

    *Peter*
  • DroneDrone Posts: 433
    edited 2007-11-05 05:59
    Ken, On the Propeller Protoboard for-example, I believe they put the 5V and 3.3V regulators in series so there isn't such a large drop between the user input voltage and 3.3V, which would generate more heat in the 3.3V regulator due to the larger voltage drop compared with being in series with the 5V regulator. The 3.3V regulator can follow the 5V regulator because it is a Low-Drop-Out (LDO) device that works with an input voltage rather close (5-3.3=1.8V) to it's output voltage rating. The obvious disadvantage of doing this is that the 5V regulator must supply all the current for both 5V and 3.3V loads.

    Regards,

    David
  • Paul Sr.Paul Sr. Posts: 435
    edited 2007-11-05 16:30
    Peter Jakacki said...
    Some members have expressed interest in the little switch-mode module that I mentioned plus one asked for 3.3V output as well. I am making these modules available at a very attractive price (cost+). Besides the 3-pin module I also have a 7-pin SIL module that features +5V@500ma switch-mode as well as a 3.3V @250ma LDO output with 500ma combined output current. There will also be a 1A version but the 0.5A part is cheaper. For propheads I am trying to keep the cost of these regulators under $10ea in quantities of 10 off.

    *Peter*

    Peter,

    Is that price for "fully populated" boards ready to run?
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